This invention relates to an acceleration sensor capable of detecting accelerations with respect to a plurality of axes.
A semiconductor acceleration sensor is an example of such an acceleration sensor. As disclosed, for example, in Japanese Laid-Open Patent Application No. Hei 4-130277, this semiconductor acceleration sensor is formed by etching a silicon substrate and includes a weight, a rectangular support surrounding this weight, and flexible and elastically deformable thin bridges for connecting the support and the weight together. The support, the weight and the bridges are integrally formed. The weight is connected at opposite ends thereof (in the direction of an X axis) to the supports through one or two pairs bridges. A pair of movable electrodes are formed on opposite surfaces of the weight in the sense of a direction of a Z axis. Disposed opposite the pair of movable electrodes are a pair of stationary electrodes. These movable electrodes and stationary electrodes constitute a pair of condensers. In this acceleration sensor, the weight is displaced in the direction of the Z axis when an acceleration is acted in the direction of the Z axis, and the bridges are expanded by elastic deformation thereof. The capacity variations of the condensers at that time indicate the direction and amount of the acceleration in the Z axis. However, this acceleration sensor is capable of detecting only the acceleration in a direction of one axis, i.e., Z axis.
A semiconductor acceleration sensor disclosed in FIGS. 1 to 6 of European Patent No. 497289 comprises a weight of a rectangular configuration, a rectangular support surrounding the weight, and a pair of bridges for connecting central parts of longitudinal opposite sides of the weight to the support. The pair of bridges are thin, flexible, and elastically deformable. In this embodiment, when an angular acceleration is applied in the sense of an X axis passing the pair of bridges, the weight is rotationally displaced about the X axis and the bridges are twisted in response to the rotational motion of the weight. The weight is stopped at a location where an elastic force corresponding to the twisting amount of the bridges and a force accompanying the angular acceleration are balanced. Accordingly, the angular acceleration can be detected by detecting an amount of the rotational displacement of the weight. Means for detecting the amount of the rotational displacement of the weight comprises four condensers. Specifically, opposite end portions of the weight in the longitudinal direction (Y axis) are provided with movable electrodes formed on opposite surfaces thereof in the direction of the Z axis. By placing stationary electrodes opposite these four movable electrodes, the four condensers are formed. In this acceleration sensor, however, only an angular acceleration about one axis, i.e., X axis, can be detected.
FIGS. 7 to 12 of the European Patent disclose a sensor capable of detecting angular accelerations about an X axis and a Y axis. This sensor comprises an inner weight having a rectangular configuration, an outer weight surrounding the inner weight, and a support surrounding the outer weight. The outer weight is supported by the support through a pair of bridges disposed on the Y axis, while the inner weight is disposed on the outer weight through a pair of bridges disposed on the X axis. The inner weight is rotationally displaced about the X axis, while the outer weight is rotationally displaced about the Y axis. This acceleration sensor is capable of detecting angular accelerations about two axes, i.e., X and Y axes, but incapable of detecting an acceleration in a Z axis.